Catalyst and process for the alkylation of aliphatic hydrocarbons with olefins

ABSTRACT

A process for the alkylation of aliphatic hydrocarbons with olefins catalyzed by a silica-based material, characterised in that the surface Si--OH groups of said silica are esterified with a fluoroalkylsulphonic acid of formula CF 3  (CF 2 ) n  SO 3  H, where n is a whole number between 0 and 11, said material having a Hammett acidity Ho≦-11.4.

This invention relates to the alkylation of aliphatic hydrocarbons inthe presence of an acid catalyst, and to the catalyst itself. Theacid-catalyzed alkylation of aliphatic hydrocarbons with olefins is awell known process in the preparation of high-octane products to beadded to gasolines for improving their characteristics.

This reaction can be conducted using strong mineral acids, in particularconcentrated sulphuric acid or anhydrous hydrofluoric acid, as catalyst.

This acids are used in excess over the feed mixture and result in theformation of oligomers and a large quantity of fluorinated andsulphonated by-products. In addition, hydrofluoric acid is a low-boilingliquid (B.P.=19° C.) which in the case of accidental escape from thereactor can form extremely corrosive acid clouds difficult to eliminate.Sulphuric acid, which requires a reaction temperature of less than 10°C. to maintain a sufficiently high product quality, forms high viscosityemulsions which are difficult to mix, and also results in the formationof large quantities of sulphonated compounds which involve costlydisposal. To avoid the aforesaid problems it has been sought to usesolid catalysts. In particular, U.S. Pat. Nos. 3,851,004, 4,384,161,3,647,916 and 5,073,665 describe the use of Y and X zeolites exchangedwith salts of metals pertaining to the lanthanum group. These catalystsdo not exhibit a sufficiently high productivity, in terms of quantity ofthe alkylated product, to make their industrial application interesting.To this must be added the short duration of the catalytic cycle and areduction in catalyst properties during successive regenerationoperations. FR 2631956 describes the use of beta zeolite in acid form asan alkylation catalyst. Again in this case the results achieved do notseem to be promising for its use at an industrial level. Catalystsystems based on Lewis acids, mainly AlCl₃ and BF₃, supported on inertoxides have also been proposed (U.S. Pat. Nos. 4,918,255, 4,384,161, WO900053, WO 900054, U.S. Pat. Nos. 3,855,343, 4,935,577 and 5,012,039).The best results seem to be obtained with boron trifluoride with addedtraces of water, but problems remain regarding the system stabilityunder the working conditions, due mainly to active phase loss.

It is also known to use trifluoromethanesulphonic acid as a catalyst forhydrocarbon alkylation with olefins. U.S. Pat. Nos. 3,970,721,4,118,433, GB 1463320 and CA 1020590 describe the use of this acid as anadditive for sulphuric acid or hydrofluoric acid to improve theircatalytic properties. The improvement in the results obtained does notappear to be sufficient to compensate the increased costs resulting fromthe use of this costly reagent. EP 433954 describes a process forhydrocarbon alkylation with olefins which uses triflic acid adsorbed onpreviously dried silica as catalyst. This system enables an alkylate ofsufficient quality to be obtained, but the catalyst has a rather lowstability with time due to elution phenomena. In addition, the presenceof free triflic acid can easily result in corrosion phenomena.

We have now found a catalyst for hydrocarbon alkylation with olefinswhich not only exhibits excellent catalytic performance, exceeding thatachievable using triflic acid adsorbed on silica, but also possessesconsiderable stability with time, and can be easily regenerated.

The present invention therefore provides a silica-based materialcharacterised in that the surface Si--OH groups of said silica areesterified with a fluoroalkylsulphonic acid of formula CF₃ (CF₂)_(n) SO₃H, where n is a whole number between 0 and 11, said material having aHammett acidity Ho≦-11.4.

Materials with a Hammett acidity Ho <-12 constitute a preferred aspectof the present invention.

Presumably the material of the present invention carries on the silicasurface substituents of formula (I) ##STR1##

In accordance therewith, on ²⁹ Si--MAS--NMR analysis a signal isobserved at about -110 ppm (TMS reference) characteristic of bulksilicon Si--(OSi)₄, and at about -103 ppm attributable both to siliconwith a hydroxyl substituent, Si--OH, and to silicon bound to an --O--Ssubstituent, Si--O--S. The evidence for the presence of two species inthe signal at -103 ppm was provided using the ²⁹ Si--MAS--NMR method incross-polarization (CP), which highlights the NMR signal of thosesilicons spatially close to hydrogen (Si--OH type). In this respect,comparing the ²⁹ Si--CP--MAS--NMR spectra of the materials of thepresent invention with the silica precursor, a significant decrease inthe presence of Si--OH species was noted in the spectra of the newmaterials, whereas the NMR non-CP signal at --103 ppm does not decrease.This is in accordance with the formation of a chemical bond between thesilica and the fluoroalkylsulphonic acid, which reduces the surfaceSi--OH species (found in CP), to form silicon-oxygen-sulphur Si--O--Sspecies (not found in CP).

In addition, the material of the present invention shows an IR signal at1546 cm⁻¹ after treatment with pyridine. This signal, attributable tothe interaction of pyridine with a Bronsted site, is in accordance withthe presence of the acid OH group in formula (I). Materials with aHammett acidity Ho of between -11.4 and -12 also provide good catalyticperformance.

Preferably the fluoroalkylsulphonic acid which esterifies the surfaceSi--OH groups of the silica is of formula CF₃ (CF₂)_(n) SO₃ H, where nis a whole number between 0 and 5.

Even more preferably trifluoromethanesulphonic acid (triflic acid) CF₃SO₃ H is used.

In this particular case it is presumable that the following substituentsare present on the silica surface: ##STR2##

According to a preferred aspect, this material has a Hammett acidity ofbetween -13.5 and -14.

The present invention also provides the method for preparing theaforedescribed new materials. Said method consists of:

a) mixing together tetraethylsilicate and fluoroalkylsulphonic acid offormula CF₃ (CF₂)_(n) SO₃ H, where n is a whole number between 0 and 11,in aqueous solution in a CF₃ (CF₂)_(n) SO₃ H/Si(OEt)₄ weight ratio ofbetween 0.1 and 30%, to obtain a gel;

b) drying the gel obtained;

c) reacting the dried gel with thionyl chloride in a SOCl₂ /gel weightratio of between 100 and 1;

d) removing the excess thionyl chloride by distillation.

Stage a) consisting of hydrolysis of the tetraethylsilicate by thefluoroalkylsulphonic acid is conducted at a temperature between ambientand 100° C. The preferred procedure is to dilute thefluoroalkylsulphonic acid in distilled water heated to the chosentemperature and then add the tetraethylsilicate.

The fluoroalkylsulphonic acid used is preferably of formula CF₃(CF₂)_(n) SO₃ H, where n is a whole number between 0 and 5. Even morepreferably trifluoro-methanesulphonic acid (triflic acid) CF₃ SO₃ H isused. The time for complete gelling varies depending on the temperatureand on the concentrations, and is normally within the range of 0.5-24hours.

Conveniently, in the reaction mixture the CF₃ (CF₂)_(n) SO₃ H/Si(OEt)₄weight ratio is chosen within the range of 5-20%. The gel formedfollowing hydrolysis is dried in stage b) under vacuum at a temperatureof between 20° and 80° C., preferably 50°-60° C., for 12-48 hours.

The dried gel is subjected in stage c) to dehydration with thionylchloride. The reaction is conducted at a temperature of between 20° and50° C. for 4-12 hours. The thionyl chloride is then removed bydistilling it off under vacuum, preferably at ambient temperature.

The catalyst of the present invention can also be conveniently preparedby a process of thermal type. This process, which represents a furtheraspect of the present invention, consists of thermally treatinganhydrous silica and fluoroalkylsulphonic acid CF₃ (CF₂)_(n) SO₃ H,where n is between 0 and 11, in a CF₃ (CF₂)_(n) SO₃ H/SiO₂ weight ratioof between 0.1 and 30%, and preferably between 5 and 20%, at atemperature of between 50° and 300° C., preferably between 100° and 200°C., for 12-48 hours. It is convenient to operate at a temperature closeto the boiling point of the acid. The anhydrous silica is prepared bydrying at a temperature of about 400° C. for 12-48 hours. Thefluoroalkylsulphonic acid used is preferably of formula CF₃ (CF₂)_(n)SO₃ H, where n is a whole number between 0 and 5. Even more preferablytrifluoromethanesulphonic acid is used.

According to a further synthesis method, which facilitates the obtainingof a catalyst according to the present invention with uniformdistribution of the active phase, fluoroalkylsulphonic acid CF₃(CF₂)_(n) SO₃ H is dissolved in a solvent, mixed with anhydrous silicain a CF₃ (CF₂)_(n) SO₃ H/SiO₂ weight ratio of between 0.1 and 30%, thesolvent is then removed by stripping, possibly under vacuum, and thesystem subjected to thermal treatment at a temperature of between 50°and 300° C. for 12-48 hours.

Solvents suitable for this purpose are those solvents inert towards bothfluoroalkylsulphonic acid and silica which have a boiling point lowerthan that of fluoroalkylsulphonic acid so that they can be easilyremoved by stripping, possibly under hot conditions.

Suitable solvents include for example water, freons, trifluoroaceticacid and trifluoroacetic anhydride. The freon1,1,2-trichloro-2,2,1-trifluoroethane is preferably used.

The catalyst of the present invention is active in the alkylation ofaliphatic hydrocarbons with olefins, it exhibits superior performancethan triflic acid adsorbed on silica, it is very stable with time underthe reaction conditions, and is easily regenerated.

A further aspect of the present invention is therefore a process for thealkylation of aliphatic hydrocarbons with olefins in the presence of asilica-based catalyst, characterised in that the surface Si--OH groupsof said silica are esterified with a fluoroalkylsulphonic acid offormula CF₃ (CF₂)_(n) SO₃ H, where n is a whole number between 0 and 11,said material having a Hammett acidity Ho≦-11.4.

The use of materials with a Hammett acidity Ho<-12 constitutes apreferred aspect of the present invention.

Preferably the fluoroalkylsulphonic acid used to esterify the surfaceSi--OH groups of the silica is of formula CF₃ (CF₂)_(n) SO₃ H, where nis a whole number between 0 and 5.

Even more preferably trifluoromethanesulphonic acid (triflic acid) CF₃SO₃ H is used.

This process consists of alkylating an aliphatic hydrocarbon substratewith an alkylating agent of olefinic type by passing the mixture ofhydrocarbon and olefin under alkylation conditions through a reactorwith a fixed catalyst bed.

The alkylation reaction is conducted at a temperature of between -20°and 100° C. at a pressure of between 5 and 40 atm and an LHSV spacevelocity of between 0.1 and 10 h⁻¹. The operating temperature isconveniently close to ambient, with a space velocity of between 2 and 4h⁻¹, and preferably between 2 and 3 h⁻¹.

The hydrocarbon substrate consists of a C₄ -C₁₀ isoalkane. The substrateis preferably isobutane.

The alkylating agent is a C₂ -C₁₀ olefin. The olefin is preferably1-butene, 2-butene or their mixtures.

The weight ratio of hydrocarbon substrate to alkylating agent in themixture fed to the reactor is between 5:1 and 100:1.

The products obtained with this process are mainly trimethyl pentanes(TMP), of which the product present in greatest concentration is2,2,4-trimethylpentane (isooctane).

The catalyst of the present invention can be easily regenerated afterremoval from the reaction environment, by operating in the followingmanner:

1) washing the catalyst with water, resulting in hydrolysis of the bondsbetween the silica and fluoroalkylsulphonic acid and extraction of thefluoroalkylsulphonic acid into the aqueous phase,

2) regenerating the silica by thermal treatment in air at 400°-700° C.,

3) recovering the fluoroalkylsulphonic acid from the aqueous phase bydistillation and subsequent drying, then reacting it with theregenerated silica at a temperature of between 50° and 300° C.,preferably 100°-200° C., for 12-48 hours.

The following examples are given to illustrate the present invention.

EXAMPLE 1

Catalyst preparation

72 ml of distilled water are placed in a beaker and heated to 60° C.,after which 3 ml of triflic acid are added while agitating. 42 g oftetraethylorthosilicate are added while maintaining this temperature.Complete gelling is achieved after about 2 hours, the gel obtained isdried at 50° C. for 24 hours and is then reacted with 50 ml of thionylchloride for 5 hours at 25° C.. On termination the excess thionylchloride is distilled off under vacuum at ambient temperature.

A catalyst is obtained having a surface area of 560 m² /g. By ²⁹ Si NMRanalysis and the cross-polarization technique it is found that in theproduct obtained the Si--OH groups initially present on the silicasurface have decreased to 66% of their initial value, whereas a newspecies Si--O--S (34%) has formed.

EXAMPLE 2

Catalyst preparation

11 cc of 70-230 mesh silica previously dried for 24 hours at 400° C. areplaced in a 14 ml pyrex glass vial. 1 ml of triflic acid is then drippedin, the vial is sealed and is maintained at 150° C. for 48 hours. Acatalyst is obtained having a surface area of 500 m² /g. Using ²⁹ Si NMRanalysis and the cross-polarization technique it is found that in theproduct obtained the Si--OH groups initially present on the silicasurface have decreased to 68% of their initial value, whereas a newspecies Si--O--S (32%) has formed.

EXAMPLE 3

Catalyst preparation

50 ml of a solution consisting of 2 ml of triflic acid in 48 ml ofanhydrous 1,1,2-trichloro-2,2,1-trifluoroethane are placed in apreviously flame-heated flask under nitrogen, and 20 ml of 70-100 meshsilica previously treated at 400° C. for 24 hours are added to thissolution.

The system is kept under agitation for 30 min, after which the solventis stripped off under vacuum for 3 h. After this time period the totaldisappearance of the NMR signal attributable to the fluorine present inthe freon molecule is noted in the solid residue.

The solid consisting of silica and triflic acid is then placed in aglass vial which is sealed and maintained at 165° C. for 24 h.

EXAMPLE 4

Catalyst preparation

A solution consisting of 50 ml of trifluoroacetic acid and 2 ml oftriflic acid are placed in a previously flame-heated 100 ml flask, and20 ml of silica previously treated at 400° C. for 24 hours are added.

The solution is then agitated for 30 min, after which thetrifluoroacetic acid is distilled off under vacuum at 70° C. for 5 h.

An acidimetric titration is then conducted on the residual solid, theacidity concentration found being equal to 100% of the triflic acidcontent.

The solid consisting of silica and triflic acid is then placed in aglass vial which is sealed and maintained at 165° C. for 24 h.

EXAMPLE 5

Catalyst preparation

A solution consisting of 10 ml of distilled water and 2 ml of triflicacid is placed in a 100 ml flask, and 10 ml of 70-100 mesh silica areadded.

The suspension is dried under vacuum at a temperature of 100° C. for 12hours.

After this time period a product of Ho=-11.4 is obtained.

EXAMPLE 6

11 cc of catalyst prepared in accordance with Example 1 are placed in areactor of diameter 0.76 cm and length 26 cm.

A mixture containing isobutane as substrate and 1-butene as alkylatingagent in an isobutane: 1-butene ratio of 20:1 is fed to the reactor at atemperature of 25° C., a pressure of 17 bar and a flow rate of 0.9ml/min. The results obtained are shown in the following table:

    ______________________________________                                        C.sub.8 yield                   Time                                          % molar  Conversion    TMP %    (min)                                         ______________________________________                                        80       99            76       20                                            94       99            76       60                                            99       99            76       360                                           ______________________________________                                    

TMP% indicates the trimethylpentane selectivity.

Triflic acid does not appear in the effluent.

EXAMPLE 7

11 cc of catalyst prepared in accordance with Example 2 are placed in areactor of diameter 0.76 cm and length 26 cm.

A mixture containing isobutane as substrate and 1-butene as alkylatingagent in an isobutane: 1-butene ratio of 20:1 is fed to the reactor at atemperature of 25° C., a pressure of 17 bar and a flow rate of 0.9ml/min. The results obtained are shown in the following table:

    ______________________________________                                        C.sub.8 yield                   Time                                          % molar  Conversion    TMP %    (hours)                                       ______________________________________                                        99       99            68       1                                             95       99            68       3                                             79       99            69       24                                            ______________________________________                                    

TMP% indicates the trimethylpentane selectivity.

Triflic acid does not appear in the effluent.

EXAMPLE 8

11 cc of catalyst prepared in accordance with Example 3 are placed in areactor of diameter 0.76 cm and length 26 cm.

A mixture containing isobutane as substrate and 1-butene as alkylatingagent in an isobutane: 1-butene ratio of 20:1 is fed to the reactor at atemperature of 25° C., a pressure of 24 bar and a flow rate of 0.6ml/min. The results obtained are shown in the following table:

    ______________________________________                                        C.sub.8 yield                   Time                                          % molar  Conversion    TMP %    (hours)                                       ______________________________________                                        89       99            66       1                                             95       98            60       3                                             95       98            66       6                                             ______________________________________                                    

TMP% indicates the trimethylpentane selectivity.

Triflic acid does not appear in the effluent.

EXAMPLE 9

10 cc of catalyst prepared in accordance with Example 4 are placed in areactor of diameter 0.76 cm and length 26 cm.

A mixture containing isobutane as substrate and 1-butene as alkylatingagent in an isobutane: 1-butene ratio of 20:1 is fed to the reactor at atemperature of 25° C., a pressure of 24 bar and a flow rate of 0.6ml/min. The results obtained are shown in the following table:

    ______________________________________                                        C.sub.8 yield                   Time                                          % molar  Conversion    TMP %    (hours)                                       ______________________________________                                        78       98            86       0.5                                           95       96            86       2                                             98       98            84       5                                             ______________________________________                                    

TMP% indicates the trimethylpentane selectivity.

No emergence of triflic acid is observed within a period of 5 hours.

EXAMPLE 10

2.2 cc of catalyst prepared in accordance with Example 5 are placed in areactor of diameter 0.76 cm and length 26 cm. A mixture containingisobutane as substrate and 1-butene as alkylating agent in an isobutane:1-butene ratio of 20:1 is fed to the reactor at a temperature of 25° C.,a pressure of 20 bar and a flow rate of 0.11 ml/min. The resultsobtained are shown in the following table:

    ______________________________________                                        C.sub.8 yield                   Time                                          % molar  Conversion    TMP %    (hours)                                       ______________________________________                                        95       99            79       0.25                                          98       99            66       1                                             ______________________________________                                    

TMP% indicates the trimethylpentane selectivity.

No emergence of triflic acid is observed within a period of one hour.

EXAMPLE 11 (Comparative)

The following example is conducted in accordance with the processdescribed in EP433954, and shows that triflic acid adsorbed on silica isless active in an alkylation reaction than the catalyst of the presentinvention. 11 cc of 70-230 mesh silica are placed in a reactor ofdiameter 0.76 cm and length 26 cm, and heated for 2 hours in a drynitrogen stream to 400° C. It is cooled under nitrogen, 1 ml of triflicacid are added and a mixture containing isobutane and 1-butene in anisobutane: 1-butene ratio of 20:1 is then fed to the reactor at atemperature of 25° C., a pressure of 17 bar and a flow rate of 0.9ml/min. The results obtained are shown in the following table:

    ______________________________________                                        C.sub.8 yield                   Time                                          % molar  Conversion    TMP %    (min)                                         ______________________________________                                        52       99            70       20                                            61       99            71       60                                            62       99            71       180                                           ______________________________________                                    

TMP% indicates the trimethylpentane selectivity.

Emergence of triflic acid is observed after 3 hours and 30 minutes. Theobserved elution time depends on the length of the silica bed. Elutionis practically immediate if the triflic acid is distributed from thebeginning over all the reactor silica.

We claim:
 1. A catalyst consisting of silica, characterized in that thesurface Si--OH groups of said silica are esterified with afluoroalkylsulphonic acid of formula CF₃ (CF₂)_(n) SO₃ H, wherein n is awhole number between 0 and 11, said catalyst having:a) a Hammett acidityHo≦-11.4 b) an IR signal at 1546 cm⁻¹ after treatment with pyridine. 2.A catalyst in accordance with claim 1, having a Hammett acidity Ho<-12.3. A catalyst in accordance with claim 1, wherein the Si--OH groups areesterified by a fluoroalkylsulphonic acid of formula CF₃ (CF₂)_(n) SO₃H, where n is a whole number between 0 and
 5. 4. A catalyst inaccordance with claim 3, wherein the fluoroalkylsulphonic acid istrifluoromethanesulphonic acid.
 5. A catalyst in accordance with claim 4having a Hammett acidity Ho of between -13.5 and -14.
 6. A method forpreparing a silica containing catalyst characterised in that the surfaceSi--OH groups of said silica are esterified with a fluoroalkylsulphonicacid of formula CF₃ (CF₂)_(n) SO₃ H, wherein n is a whole number between0 and 11, said catalyst having a Hammett acidity Ho<-11.4 and an IRsignal at 1546 cm⁻¹ after treatment with pyridine, consisting of thesteps of:a) mixing together tetraethylorthosilicate andfluoroalkylsulphonic acid of formula CF₃ (CF₂)_(n) SO₃ H, wherein n is awhole number between 0 and 11, in aqueous solution in a CF₃ (CF₂)_(n)SO₃ H/Si(OEt)₄ weight ratio of between 0.001 and 0.3, to obtain a gel;b) drying the gel obtained; c) reacting the dried gel with thionylchloride in a SOCl₂ /gel weight ratio of between 100 and 1; and d)removing the excess thionyl chloride by distillation.
 7. A method inaccordance with claim 6, wherein in stage a) the CF₃ (CF₂)_(n) SO₃H/Si(OEt)₄ weight ratio lies within the range of 0.05 and 0.2.
 8. Amethod in accordance with claim 6, wherein stage a) is conducted at atemperature between ambient and 100° C. for a time of 0.5-24 hours.
 9. Amethod in accordance with claim 6, wherein in stage b) the gel is driedunder vacuum at a temperature of between 20° and 80° C. for 12-48 hours.10. A method in accordance with claim 6, wherein stage c) is conductedat a temperature of between 20° and 50° C. for 4-12 hours.
 11. A methodin accordance with claim 6, wherein the distillation of stage d) isconducted under vacuum at ambient temperature.
 12. A process forpreparing a silica containing catalyst characterised in that the surfaceSi--OH groups of said silica are esterified with a fluoroalkylsulphonicacid of formula CF₃ (CF₂)_(n) SO₃ H, wherein n is a whole number between0 and 11, said catalyst having a Hammett acidity Ho<-11.4 and an IRsignal at 1546 cm⁻¹ after treatment with pyridine, consisting ofthermally treating a fluoroalkylsulphonic acid of formula CF₃ (CF₂)_(n)SO₃ H, where n is a whole number between 0 and 11, and anhydrous silicain a CF₃ (CF₂)_(n) SO₃ H/SiO₂ weight ratio of between 0.001 and 0.3, ata temperature of between 50° and 300° C. for 12-48 hours.
 13. A processin accordance with claim 12, wherein the temperature is between 100° and200° C.
 14. A process in accordance with claim 12, wherein the weightratio of fluoroalkylsulphonic acid to anhydrous silica is between 0.05and 0.2.
 15. A process in accordance with claim 12, wherein theanhydrous silica is prepared by drying silica at a temperature of about400° C. for 12-48 hours.
 16. A process for preparing a silica containingcatalyst characterised in that the surface Si--OH groups of said silicaare esterified with a fluoroalkylsulphonic acid of formula CF₃ (CF₂)_(n)SO₃ H, wherein n is a whole number between 0 and 11, said catalysthaving a Hammett acidity Ho<-11.4 and an IR signal at 1546 cm⁻¹ aftertreatment with pyridine, comprising the steps of: dissolving thefluoroalkylsulphonic acid CF₃ (CF₂)_(n) SO₃ H in a solvent, mixing withanhydrous silica in a CF₃ (CF₂)_(n) SO₃ H/SiO₂ weight ration of between0.001 and 0.3, removing the solvent under vacuum, and subsequentlyheating the resulting product at a temperature of between 50° and 300°C. for 12-48 hours.
 17. A process in accordance with claim 16, whereinthe solvent is selected from the group consisting of water, freons,trifluoroacetic acid and trifluoroacetic anhydride.
 18. A process inaccordance with claim 17, wherein the solvent is1,1,2-trichloro-2,2,1-trifluoroethane.